Digital printing technology has made it possible to implement digital, i.e., bit map addressable, printing for the purpose of evidencing payment of postage by a postage-meter-like device. In order to distinguish such postage-meter-like devices from the typical postage meter which uses mechanical printing techniques, such devices will be named herein Postage Evidencing Devices or PED's. The computer driven printer can print the postal indicia in a desired location on the face of a mail piece. As used herein the postal indicia includes a Postal Revenue Block or PRB. The PRB typically contains data such as the postage value, a unique PED identification number, the date and in some applications the name of the place where the mail is originating.
From the Post Office's point of view, it will be appreciated that the digital printing makes it fairly easy to counterfeit an indicia with the PRB since any suitable computer may be used to generate multiple images.
It is known to prevent such counterfeiting by including certain information in the block in both plain text and cipher text. For example the postage amount, date and sequential piece count can be encrypted using either a secret or public key encryption algorithm and printed along with the plain text counterpart in the PRB. The value of the franking used in the encryption can be determined from the encryption to learn whether the value as printed on the mailpiece is correct. See, for example, U.S. Pat. Nos. 4,757,537 and 4,775,246 to Edelmann et al. as well as U.S. Pat. No. 4,649,266 to Eckert. It is also known to authenticate a mailpiece by including the address as a further part of the encryption as described in U.S. Pat. No. 4,725,718 to Sansone et al. and U.S. Pat. No. 4,743,747 to Fougere et al. The encrypted messages may be included in graphic form as disclosed in U.S. Pat. Nos. 4,835,713 and 4,949,381 to Pastor.
U.S. Pat. No. 5,075,862 to Doeberl et al. discloses a metering system which includes changes in the indicia, such as the printing or not printing of a star based on a suitable algorithm involving date and postage amount, for authentication the printing. U.S. Pat. No. 4,913,912 to Chickneas shows a secure meter separated from a printhead which prints a validation number at each printing for authorization of the printing.
U.S. Pat. Nos. 4,637,051, to Clark; 4,641,346 to Clark et al.; 4,829,568 to Clark et al.: and 4,660,221 to Dlugos teach the printing of indicia in human readable format wherein the dots forming the indicia are displaced in accordance with an a coded message which may be read with an overlay. The coded message may also be printed in barcode.
U.S. Pat. No. 5,186,498 to Dietrich discloses the forming of a characteristic identification pattern to be printed which includes an encrypted number representative of the machine and the fee amount and date of printing.
To authenticate a mailpiece using many of the foregoing encryption techniques, the verifying Authority must again generate the encryption from the plain text using the identical key used by the purported mailer. If the printed cipher text matches or if the graphical pattern matches an overlay, the mailpiece is verified. If there is no match then appropriate action may be initiated. Others require that a template be used to determine the encryption.